Treatment of a thiosulfate solution with hydrogen sulfide

ABSTRACT

An aqueous stream containing a water-soluble thiosulfate compound is treated to substantially lower the thiosulfate content thereof and to produce elemental sulfur by the steps of: (a) reacting the aqueous stream with an amount of HsS selected to provide greater than 2 moles of H2S per mole of thiosulfate at polysulfide formation conditions selected to form an effluent stream containing the corresponding polysulfide compound and unreacted H2S; (b) rapidly cooling the total effluent stream from step (a) to a temperature effective to decompose at least a portion of the resulting polysulfide compound and form elemental sulfur without substantially changing the pressure of the effluent stream; (c) separating sulfur from the resulting cooled effluent stream without changing the pressure of same; and (d) reducing the pressure of the resulting cooled and substantially sulfur-free effluent stream to yield a treated water stream having a substantially reduced thiosulfate content and an H2Scontaining gas stream. Principal utility of the disclosed treatment procedure is associated with the clean-up or regeneration of aqueous streams containing undesired thiosulfate compounds so that they can be reused in the process which originally produced them or discharge into a suitable waste water sewer or stream without causing a major pollution problem. Key features of the disclosed method are: (1) operation of the reaction step with excess amounts of hydrogen sulfide so that unreacted hydrogen sulfide is present in the effluent therefrom; (2) rapid cooling of the total effluent stream from the reaction step under a positive hydrogen sulfide partial pressure; and (3) separation of H2S from the product stream only after the separation of sulfur.

United States Patent [191 Urban Nov. 20, 1973 TREATMENT OF A THIOSULFATESOLUTION WITH HYDROGEN SULFIDE [75] Inventor: PeterUrban,Northbrook,lll.

[73] Assignee: Universal Oil Products Company,

Des Plaines, Ill.

22 Filed: Junel6, 1972 21 Appl. No.: 263,509

[52] U.S. Cl 210/50, 210/60, 423/562, 423/573 [51] Int. Cl. C02c 5/04[58] Field of Search 210/50, 60; 423/562, 423/573 [56] References CitedUNITED STATES PATENTS 3,536,618 10/1970 Urban et al 210/50 PrimaryExaminer-Michael Rogers AttorneyJames R. Hoatson, Jr. et al.

[5 7] ABSTRACT An aqueous stream containing a water-soluble thiosulfatecompound is treated to substantially lower the thiosulfate contentthereof and to produce elemental sulfur by the steps of: (a) reactingthe aqueous stream with an amount of H 8 selected to provide greaterthan 2 moles of H 8 per mole of thiosulfate at polysulfide formationconditions selected to form an efi'luent stream containing thecorresponding polysulfide compound and 'unreacted H 8; (b) rapidlycooling the total effluent stream from step (a) to a temperatureeffective to decompose at least a portion of the resulting polysulfidecompound and form elemental sulfur without substantially changing thepressure of the effluent stream; (0) separating sulfur from theresulting cooled efi'luent stream without changing the pressure of same;and (d) reducing the pressure of the resulting cooled and substantiallysulfur-free effluent stream to yield a treated water stream having asubstantially reduced thiosulfate content and an H S-containing gasstream. Principal utility of the disclosed treatment procedure isassociated with the clean-up or regeneration of aqueous streamscontaining undesired thiosulfate compounds so that they can be reused inthe process which originally produced them or discharge into a suitablewaste water sewer or stream without causing a major pollution problem.Key features of the disclosed method are: (1) operation of the reactionstep with excess amounts of hydrogen sulfide so that unreacted hydrogensulfide is present in the effluent therefrom; (2) rapid cooling of thetotal effluent stream from the reaction step under a positive hydrogensulfide partial pressure; and (3) separation of H 8 from the productstream only after the separation of sulfur.

12 Claims, No Drawings TREATMENT OF A THIOSULFATE SOLUTION WITH HYDROGENSULFIDE The subject of the present invention is a multi-step method forthe selective treatment of an aqueous stream containing a water-solublethiosulfate compound in order to convert the thiosulfate compound toelemental sulfur and to lower the total sulfur content (i.e., the totalamount of sulfur contained in the solution in any form, calculated on anelemental sulfur basis) of the aqueous stream to the point where it canbe reused in the process in which it originated or it can be safelydischarged into a suitable sewer without causing a major pollutionproblem. More precisely, the present invention involves a novelfour-step method for treating an aqueous stream containing awater-soluble thiosulfate compound wherein the thiosulfate compound isfirst reacted with H S under conditions selected to result in arelatively rich polysulfide compound, the resulting polysulfide compoundis at least partially decomposed by rapid cooling under a positive H 8partial pressure, sulfur is then recovered from the resulting cooledsolution without substantially changing the pressure of the effluentstream, and finally H 8 is separated from the resulting cooled andsubstantially elemental sulfur-free effluent stream at a reducedpressure to form a treated water stream of greatly reduced thiosulfatecontent. In one specific aspect the present invention involves a methodfor the treatment of an aqueous stream containing sodium thiosulfate toproduce elemental sulfur and treated aqueous stream, which has a greatlyreduced total sulfur content relative to the inputthiosulfate-containing aqueous stream, by a procedure which involves theproduction of an effluent stream containing a relatively rich sodiumpolysulfide compound and unreacted H 8 effluent stream containing arelatively rich sodium polysulfide compound and unreacted H 8 by areaction between H 8 and the input solution and the subsequentdecomposition of same to yield elemental sulfur by means of a coolingstep and a sulfur separation step both of which are conducted under apositive H partial pressure.. In another important aspect, the presentinvention relates to the treatment of a rich absorbent stream (which isproduced by scrubbing a gas stream containing sulfur dioxide with anaqueous absorbent containing an alkaline reagent and a reducing agent,selected from the group consisting of finely divided sulfur, awater-soluble sulfide compound, a polysulfide compound and mixturesthereof, at conditions selected to form a rich absorbent streamcontaining substantial amounts of the corresponding thiosulfatecompound) in order to convert the thiosulfate compound contained thereinin a selective manner to elemental sulfur and to regenerate the aqueousabsorbent solution so that it can be reused to absorb additionalquantities of sulfur dioxide.

As part of the price that has to be paid for a modern industrialsociety, large quantities of aqueous solutions of thiosulfate compoundsare currently produced in a number of industrial processes. Inparticular, aqueous solutions containing ammonium thiosulfate are anundesired side product of many economically significant industrialprocesses in the chemical, petroleum, natural gas, paper and steelindustries. For instance, in the petroleum industry an aqueous solutioncontaining ammonium thiosulfate is produced as a drag stream from sulfurrecovery systems that employ an oxidation method to recover sulfur fromammonium hydrosulfide solutions, which are commonly available as sidestreams from such typical refinery processes as hydrorefining,hydrocracking, catalytic cracking, etc. Another source of thesethiosulfate-containing streams are processes for natural gas sweetening,coal gas purfication, town gas purification, and the like processeswherein hydrogen sulfide is scrubbed from a gaseous mixture containingsame, and thereafter oxidized to elemental sulfur in a regenerationstep. In these latter types of processes, an inevitable side reactionappears to be one leading to the formation of a thiosulfate salt whichcan then accumulate in the absorbent solution. The net amount of thethiosulfate by-product being produced must then be continuously orperiodically purged from the system by discarding a drag stream. Forexample, in the Ferrox process for nautral gas sweetening or for coalgas purification, where iron oxides suspended in an alkaline, aqueoussolutions are used to extract H 8 from the gas stream and where the usedscrubbing solution is regenerated by air oxidation, formation of athiosulfate salt is observed as by-product in the regeneration step, anda drag stream containing this salt must be periodically discarded.Another pertinent example is in the Thyloxprocess. This process istypically utilized for coke-oven gas treating and employs a treatingsolution comprising arsenic trioxide and sodium carbonate dissolved inwater. The regeneration of the rich scrubbing solution is by airoxidation in a separate oxidizing zone. Once again, a drag streamcontaining thiosulfate salts and water-soluble thiocyanate salts isremoved from this process in order to purge the net make of thiosulfate,and this stream results in a requirement for continuous replacement ofsodium carbonate and arsenic trioxide. Yet another example is the Peroxprocess which utilizes an aqueous ammonium solution containing anorganic oxidation catalyst and which regenerates the rich solution byoxidation with air with consequential thiosulfate byproduct formationand requirement for a thiosulfatecontaining drag stream.

The production of an aqueous stream containing a water-solublethiosulfate compound can also be accomplished during the course of aprocess for removing S0 from a gas stream containing same. In my U. S.Pat. No. 3,644,087 I have previously disclosed how an SO scrubbing stepcan be operated to produce a rich absorbent stream containingsubstantial amounts of a watersoluble thiosulfate compound. Inparticular, I have demonstrated how the operation of aconventional SO2-scrubbing step which uses an aqueous absorbent containing a conventionalalkaline reagent such as the carbonate, bicarbonate sulfite or hydroxidesalts of ammonia, the alkali metals or the alkaline earth metals can bemodified to suppress the production of undesired, intractable sulfateby-products by the operation of the scrubbing step with the continuousinjection of a reducing agent, selected from the group consisting offinely divided elemental sulfur, a water-soluble sulfide compound, apolysulfide compound and mixtures thereof, under thiosulfate productionconditions effective to produce a rich absorbent stream containingsubstantial amounts of a water-soluble thiosulfate compound. The richabsorbent stream recovered from such a modified S0 -scrubbing method isa particularly preferred input stream for the method of the presentinvention.

Regardless of the source of the aqueous stream containing a thiosulfatecompound, it is clear that there is a substantial need for a method oftreating the thiosulfate solution in order to remove the thiosulfatecompound and allow either the reuse of the resulting treated aqueousstream in the process which produced it or the safe discharge of theresulting stream in sewers and/or rivers and streams. The firstalternative is particularly advantageous when thiosulfate-containingstream also contains other valuable reagents such as in the Thyloxprocess previously mentioned wherein the drag stream also containssodium carbonate and arsenic trioxide. In addition, the growingsensitivity of the public to the" adverse effects of indiscriminatedischarge of waste stream by the chemical and petroleum industryprovides an additional incentive for treating thesethiosulfate-containing streams prior to their discharge into sewersystems.

In a case of particular interest, hydrorefining or hydrocracking ofpetroleum distillates containing nitrogenous and-sulfurous contaminants,large quantities of ammonia and hydrogen sulfide are present in theeffluent from the hydrocarbon conversion zone. These contaminants aregenerally absorbed in an aqueous absorbent solution which is injectedinto the effluent train of condensers and separating zones associatedwith the hydrocarbon conversion process. This results in an aqueousstream containing ammonium hydrosulfide (NH HS). As discloed in my U. S.Pat. Nos. 3,530,063; 3,531,395; 3,536,618; and 3,536,619, this streamcan thereafter be subjected to an oxidizing step in order to recoversulfur therefrom or to reduce the biochemical oxygen demand thereof.Despite stringent precautions a minor amount of thiosulfate salt (i. e,(NVHQ 25 03) is inevitably formed as a side product in this oxidationstep. The resulting ammonium thiosulfate-containing aqueous solutionwithdrawn as effluent from this oxidation step cannot be directly reusedto recover an additional portion of ammonium hydrosulfide because, if itis injected into the effluent train associated with the hydrorefining orhydrocracking process, the hydrogen sulfide and/or ammonium hydrosulfidepresent in this effluent can react with the ammonium thiosulfate toproduce free sulfur which can contaminate the hydrocarbon product fromthis process leading to severe corrosion problems in downstreamequipment. Accordingly, there is a substantial need for a method oftreating an aqueous solution containing ammonium thiosulfate compound inorder to allow reuse of the treated aqueous stream within the processwhich produced it.

Quite understandably in recent years, attention has been focused upon asearch for a method of treating such an aqueous solution in order toconnect the thiosulfate compounds contained therein into products thateither can be easily separated from the treated aqueous stream or thatdo not adversely affect the characteristics of the treated water streamwhen it is discharged or reused, thereby allowing a reuse of the treatedaqueous stream in the process wherein it originated or, if desired, itsdischarge into any suitable and available sewer such as rivers, lakesand streams. It has heretofore been proposed to treat these aqueousstreams containing a thiosulfate compound with hydrogen sulfide underreduction conditions effective to produce a solution containing awater-soluble polysulfide compound with subsequent recovery of elementalsulfur by decomposition of the polysulfide compound. I have attempted toperfect such a procedure and I have noticed that if the thiosulfate isconverted to polysulfide by a reaction with H 8 and an attempt is madeto decompose the resulting polysulfide compound by conventionalacidification procedures such as stripping with carbon dioxide,substantial loss of the elemental sulfur product is encountered duringthe polysulfide decomposition step. Without the intention of being boundby any theoretical explanation of this phenomenon, I attribute this lossof elemental sulfur product that occurs in a conventional polysulfidedecomposition procedure with carbon dioxide to the back hydrolysis ofelemental sulfur product due to a reaction between the freshly formedelemental sulfur and the carbonate and bicarbonate salts formed as thepolysulfide decomposition proceeds. In essence I believe that asignificant portion of the elemental sulfur present in the polysulfidesolution recovered from the H 8 reduction step is lost during thedecomposition step due to interaction of the elemental sulfur with thebasic cations (or carbonate salts thereof) that are released during theactual decomposition reaction. As will be shown in an example, myexperiments on this phenomenon have shown that 40 to 50 percent of theelemental sulfur present in the solution charged to the decompositionstep in the form of polysulfide sulfur can be hydrolyzed therein towater-soluble sulfur products (i.e., principally back to thiosulfate)with resulting substantial decrease in yield of elemental sulfur.

The problem addressed by the present invention is then to provide amethod for treating these thiosulfatecontaining water streams and forrecovering elemental sulfur therefrom which method substantiallycontrols the loss of elemental sulfur product by back hydrolysis of thesulfur product with basic constituents of the treated water stream.

As a result of my investigations of this sulfur yield loss problem, Ihave formulated a procedure for treating an aqueous stream containing awater-soluble thiosulfate compound which substantially avoids theinteraction of the sulfur product with the basic constituents of thetreated aqueous stream, thereby enabling a sig nificant increase in theamount of elemental sulfur which can be recovered from such a solutionby treatment with H 8. The concept of the present invention isessentially based on my finding that cooling of the effluent stream fromthe H 8 reduction step is effective to decompose polysulfide andsuppress the hydrolysis of elemental sulfur if the cooling is conductedunder a positive partial pressure of H 8 and if it accomplished in arelatively rapid manner so that the polysulfide solution does not have achance to undergo a stabilization reaction. That is, my experiments haveshown that there is a short time period during which the polysulfidesolution recovered from the H 8 reduction step can be selectivelydecomposed by cooling if the cooling is performed under an H 8 partialpressure of at least 50 psi. and if the polysulfide has not had time toset to an intractable solution. The duration of this induction period isa function of the type of polysulfide compound involved, itsconcentration and the other constitutents of the solution; but ingeneral, my finding is that the period is less than one half hour, andpreferably less than 10 minutes. Coupled with this finding of aninduction period is my additional observation that the sulfur product,once it is formed by the instant cooling procedure, must be separatedfrom the cooled solution under a partial pressure of hH S. It is onlyafter this sulfur separation step that unreacted H 8 can be safelyseparated from the resulting treated stream. Thus, the essential pointof the present method involves reacting the thiosulfate-containingaqueous solution with sufficient H 8 under conditions selected to forman effluent stream containing the corresponding polysulfide compound andunreacted H 8, followed by rapid cooling of the resulting efiluentstream to a temperature where polysulfide decomposes to yield elementalsulfur and separation of the elemental sulfur from the resulting cooledsolution before the separation of unreacted H 8.

It is accordingly, an object of the present invention to provide asimple, effective and selective method for treating a water streamcontaining a water-soluble thiosulfate compound to produce elementalsulfur and a treated water stream of greatly reduced total thiosulfatecontent while minimizing the loss of elemental sulfur in the polysulfidedecomposition step due to back hydrolysis. Another object is to providea simple method for purifying thiosulfate-containing waste water streamsso that they can be reused if desired. Still another object is toprovide a method of controlling a source of potential water pollution bychemical, petroleum, steel, paper and the like industries that producethiosulfate-containing waste water streams.

In brief summary, my invention, in one embodiment, is a method fortreating an input aqueous stream containing a water-soluble thiosulfatecompound in order to substantially lower the thiosulfate content thereofand to produce elemental sulfur. The initial step of my method is areduction step and involves reacting the aqueous stream with an amountof H 8 selected to provide greater than 2 moles of H 8 per mole ofthiosulfate contained in the aqueous stream. This reduction step isperformed at polysulfide formation conditions, including a temperatureabove the melting point of sulfur and a pressure sufficient to maintainthe aqueous stream in the liquid phase, selected to form an effluentstream containing the corresponding polysulfide compound and unreacted H8. In the next step, the total effluent stream from'the reduction stepis rapidly cooled to a temperature selected to decompose at least aportion of the resulting polysulfide compound and to form elementalsulfur. This cooling step is conducted without substantially changingthe pressure of the effluent stream in order to maintain a positivepartial pressure of H 8 on same. The resulting cooled effluent stream isthen subjected to a sulfur separation step designed to remove theresulting elemental sulfur therefrom. This sulfur separation step isperformed without substantially changing the pressure of the effluentstream. in the final step the pressure of the resulting cooled andseparated effluent stream recovered from the sulfur separation step isreduced and it is separated in a gasliquid separation step into an Hs-containing gas stream and a treated water stream having'substantiallyreduced thiosulfate content.

In another embodiment, the present invention is a method as outlinedabove in the first embodiment wherein the mole ratio of H 8 tothiosulfate utilized in the reduction step is selected from the rangecorresponding to about 3.521 to about 10:1.

In yet another embodiment the present invention is a method as describedabove in the first embodiment wherein the temperature utilized in thecooling step is about 25 to about C. below the temperature used in thereduction step.

In still another embodiment, the invention is a method as characterizedin the first embodiment wherein the water-soluble thiosulfate compoundcontained in the aqueous input stream is ammonium thiosulfate or analkali metal thiosulfate or an alkaline earth metal thiosulfate.

Other objects and embodiments of the present invention are hereinafterdisclosed in the following detailed discussion of the input streams,preferred conditions, preferred reactants, output streams and mechanicsassociated with the essential and preferred steps of the presentinvention.

The starting point for the instant method is a reduction reactionbetween the thiosulfate compound contained in the input aqueous streamand hydrogen sulfide. The inorganic thiosulfate compound present in theinput aqueous stream is generally present as a water-soluble salt of therelatively common base such as ammonium thiosulfate, the water-solublealkali metal thiosulfate such as sodium and potassium thiosulfate andthe alkaline earth metal thiosulfate such as calcium thiosulfate,magnesium thiosulfate, strontium thiosulfate, and barium thiosulfate. Itis, of course, understood that the thiosulfate compound present in theinput stream maybe ionized to various degrees in the aqueous solution.For purposes of the present invention, the preferred thiosulfatecompounds are ammonium thiosulfate and sodium thiosulfate. In some casesmixtures of thiosulfate compounds may also be present in the inputsolution. The amount of thiosulfate compounds contained in the inputaqueous solution may range from 0.1 wt. percent up to the solubilitylimit of the particular thiosulfate compound in water at the conditionsutilized in the reduction step. More particularly, the thiosulfatecompound may be present in this solution in amounts corresponding toabout 0.1 to about 30 wt. percent of the solution. For example,excellent results are obtained with a solution containing about 5 toabout 25 wt. percent sodium thiosulfate.

The hydrogen sulfide reactant utilized in the reduction step of thepresent invention may be derived from any suitable source. Relativelypure hydrogen sulfide is available in liquid or gaseous form as acommercial commodity in many areas of the world. Likewise, substantiallypure streams of hydrogen sulfide can be manufactured by any of thetechniques known in the art for converting either elemental sulfur or asulfurcontaining compound such as $02 to H 8 such as by a reaction of elemental sulfur or S6 with hydrogen, a suitable hydrocarbon, carbonmonoxide or the like reducing agent at a relatively high temperatureunder conditions selected to produce H 8. Similarly, elemen-' tal sulfurcan be reacted with H O under certain conditions to produce H 8 (i.e.,the reverse of the Claus reaction). An excellent source of a suitable H8 stream is the product gas stream from the regeneration section of oneof the conventional H 8 scrubbing processes such as the Girbitolprocess, the phosphate process, the phenolate process, the vacuumcarbonate process, and the like H 8 scrubbing processes wherein theregeneration step produces a relatively rich overhead gas streamcontaining H 8. Regardless of the course of the H 8 reactant, it is anessential feature of the present method that it is supplied to thereduction step in anamount in excess of the amount that is consumedtherein so that a substantial amount of unreacted H 8 appears in theeffluent from this step. Ordinarily, this means that hydrogen sulfidemust be supplied in an amount greater than 2 moles of H 8 per mole ofthiosulfate contained in the input aqueous stream, with the preferredamount selected from the range to about 3.5:] to about 10:1. It is to benoted that the unreacted H 8 present in the effluent stream from thisinitial step is recovered in ac cordance with the present inventionafter the polysulfide compound is decomposed and sulfur is separated.The resulting H s-containing gas stream is preferably recycled to thereduction step in order to supply at least a portion of the H S reactantconsumed therein.

In accordance with the present invention, the first step of the methodis a reduction step wherein the aqueous stream is reacted with hydrogensulfide at polysulfide formation conditions. These conditions include atemperature substantially above the melting point of sulfur and moreparticularly a temperature of at least 115 C., and more preferable atemperature to 150 to about 370 C. Ordinarily best results are obtainedat a temperature of about 175 to about 250 C. The pressure utilized inthis step can be any pressure sufficient to maintain the input aqueousstream in the liquid phase and generally a pressure selected from therange of about 100 to about 5,000 psig. is sufficient. Ordinarily, thecontact time of the reactant in this step is not critical and contacttimes of about 0.01 to about 1 hour are sufficient, with preferredcontact times being about 0.05 to about 0.2 hours. Insofar at pH isconcerned, it is not a material factor in the production of polysulfideexcept that it is necessary to control the pH of the input aqueousstream in a range sufficient to insure that the effluent stream fromthis step is sufficiently alkaline to allow the polysulfide compound toexist therein. Since the pH of the input solution ordinarily tends toincrease during the course of the thiosulfate reduction reaction, the pHrequirement is typically automatically satisfied. Excellent results areobtained in this step at conditions including a temperature of 150 C., apressure of 690 psig. and a contact time of 6 minutes with an inputstream containing ammonium thiosulfate. Similarly, good results havebeen obtained when the input stream containing sodium thiosulfate atconditions including a temperature of 200 C., a pressure of 790 psigwith a contact time of about minutes.

This reduction step can be carried out in any suitable manner taught inthe art for contacting a liquid stream and a gas stream. Ordinarily, thereaction zone will contain suitable means for effecting intimate contactbetween the gas stream and the liquid stream such. as baffles, plates,trays, screens, any of the known packing materials and other well-knowndevices. The thiosulfate-containing aqueous stream can be passed intothe reduction zone in either upward, radial or downward flow with thehydrogen sulfide gas stream being simultaneously introduced into thezone in concurrent flow relative to the aqueous stream. A particularly,preferred embodiment involves downflow and concurrent flow of theaqueous stream and the hydrogen sulfide stream through the reactionzone.

Although it is not essential, this reduction step can be carried outwith any suitable reduction catalyst known to be capable of acceleratingthe reduction of a thiosulfate compound with a hydrogen sulfidereactant. It is to be understood that this reaction does not require acatalyst and my findings are that perfectly good results are obtainedwithout the use of a catalyst; however, under certain circumstances acatalyst can serve both the function of promoting contact between theaqueous stream and the gas stream and of sharply accelerating the basicpolysulfide formation reaction so that it can be performed at lowertemperatures. Based on my investigations of catalyts for this reaction,I have ascertained that particularly good results are obtained with acatalyst comprising a metallic component selected from the groupconsisting of the transition metals of Groups VI and VIII of thePeriodic Table such as chromium, molybdenum, tungsten, iron, cobaltnickel, platinum, palladium and the like. This catalytic ingredient maybe utilized in the form of a water-soluble salt of the metal added tothe aqueous solution to be treated or it may be combined with a suitablecarrier material and utilized according to any of the methods known tothe art for effecting a catalytic reaction with a solid catalyst such asa moving bed type of operation, a fluidized bed type of operation or afixed bed system. I have found best results when the catalyst isutilized as a fixed bed maintained in the reaction zone by suitablesupporting screens. A preferred embodiment of the present inventioninvolves the use in the reduction step of a fixed bed of a catalystcomprising a combination of catalytically effective amounts of an oxideor sulfide of a Group VI or Group VIII transition metal combined with asuitable porous support such as any of the refractory inorganic oxidesor any of the commonly available carbonaceous materials generally usedfor this purpose. Pre ferred supports are alumina and activated carbon.In general, the metallic component is preferably combined with thecarrier material in amounts sufficient to result in a catalystcontaining about 0.01 to about 20 wt. percent of the metallic component,calculated as the elemental metal. For this particular reaction, I haveobtained good results with a catalyst comprising cobalt sulfide combinedwith alumina or activated carbon in amounts sufficient to result in acatalyst containing about 1 to about 10 wt. percent cobalt. Thisreduction catalyst can be prepared according to any of the tech niquesknown to those skilled in the catalyst formation art with animpregnation procedure with a watersoluble, decomposable compound beingpreferred.

In the embodiments of the present invention wherein a catalyst isutilized in the reduction step, suitable liquid hourly space velocitiesare selected from the range of about 0.01 to about 10 hr. with apreferred value being with 0.25 to about 2 hr.'.

Following the reduction step, the total effluent stream therefromcontaining the polysulfide compound and unreacted I-I,S, is passedthrough a suitable cooling means designed to rapidly drop thetemperature thereof to the point where polysulfide is decomposed to formsignificant amounts of elemental sulfur. One feature of the presentinvention is that this cooling step is performed relatively rapidlyafter the effluent stream is withdrawn from the first step. By the useof the expression relatively rapidly it is intended to mean that thiscooling step is performed within 30 minutes of the time that theeffluent stream is withdrawn from the reduction zone, and preferablywithin less than 10 minutes. Another feature of this cooling step isthat it is performed on the total effluent from the reduction zonewhich, because excess H S was used therein, contains unreacted H 8 in anamount sufficient to produce a substantial I-I S partial pressure. It isgenerally preferred to use sufficient excess H 8 in the reduction stepto result in a partial pressure of H 8 of at least 50 psi. Anotheressential feature of this cooling step is that the pressure of theeffluent stream from the reaction zone is not allowed to substantiallychange during the cooling operation; that is, the cooling step isperformed at substantially the same pressure (except of course for thenormal pressure drop encountered in a continuous system) reduction step.Ordinarily, the temperature to which the effluent stream is cooled isabout 25 to about 150 C. below the inlet temperature used in thereduction step, with the preferred temperature being just above themelting point of sulfur (which is generally stated to be about 115 C).If the temperature of the effluent stream during this cooling operationis maintained above the melting point of sulfur, the separation ofsulfur from the resulting solution in the subsequent sulfur separationstep will be facilitated; otherwise if solid elemental sulfur is allowedto form it is ordinarily very finely divided sulfur in a colloidal formwhich can be very difficult to separate from the treated aqueous stream.Ordinarily, the degree of polysulfide decomposition can be controlled bycarefully adjusting the temperature to which the effluent stream iscooled. Depending somewhat on the particular thiosulfate compound, theexact conditions utilized in the polysulfide formation step and thenature of the other ingredients of the input aqueous solution, thedifferential temperature drop in this cooling step effective todecompose substantially all of the polysulfide is ordinarily in therange of about 25 to about 150 C. The cooling means utilized to effectthis cooling step can be any conventional cooling means known to thoseskilled in the art, with preference shown, of course, for the designthat allow the maximum temperature drop in the effluent stream withminimum pressure drop.

After this quench step, the resulting cooled effluent stream containselemental sulfur, a water-soluble sulfide compound, such as ammoniumhydrosulfide, sodium hydrosulfide and the like, unreacted 1-1 8 andtrace amounts of unreacted thiosulfate compound. vIn cases where it isnot desired to completely decompose the polysulfide compound formed inthe reduction zone, the cooled effluent stream may contain some minoramounts of polysulfide. In accordance with the present invention, theresulting cooled effluent stream is then passed to a suitable separatingzone wherein elemental sulfur contained therein is removed withoutsubstantially changing the pressure of this stream. That is, while stillmaintaining this stream under a partial pressure of H S. In the casewhere the elemental sulfur is present in this stream in liquid form, asimple phase separation in a settling tank is ordinarially sufficient toseparate the liquid sulfur phase from the other constituents of thisstream. If the temperature drop in the cooling step has been sufficientto produce solid sulfur, the solid sulfur is separated in this stepaccording to any of the techniques known in the art for separating asolid from the liquid under pressure such as filtration, centrifugingand the like procedure. In either case, a sulfur product stream will berecovered during this step and it can be utilized for any of the knowncommercial uses of sulfur or converted to one of the sulfur-containingproducts used in industry such as sulfuric acid. Also withdrawn fromthis step is an aqueous stream comprising the cooled and substantiallyelemental sulfur-free effluent stream from the first step. This aqueousstream is a mixture of gas and liquid and it is then charged to thefinal step of the present invention, the liquid-gas separation step.

The liquid-gas separation step performed on the aqueous stream withdrawnfrom the sulfur-separation step is ordinarily accomplished by reducingthe pressure of the stream to a value approximating atmospheric andseparating the resulting stream in a conventional gas-liquid separatingzone into an l-l S-containing gas overhead stream and a treated waterbottoms stream having a substantially reduced thiosulfate content. The HS-containing overhead gas stream is withdrawn from the separating zoneand can, if desired, be passed back to the reduction step in order tofurnish at least a portion of the H 8 reactants utilized therein. Inaddition, a portion of this overhead stream can be passed to the coolingstep in order to adjust H S partial pressure at that point if desired.The treated water stream recovered from this last step is substantiallyreduced in thiosulfate content and this ordinarily means that to percentor more of the thiosulfate originally present therein has been removed.For example, operation of the present invention on a water streamcontaining sodium thiosulfate results in the reduction of thethiosulfate content of the water stream by 93 percent is subsequentlyexplained in one of the examples. The treated water stream, whilecontaining substantially less thiosulfate, will contain a water-solublesulfide compound due to release of basic cations during the thiosulfatereduction procedure for example, when the input aqueous stream containsammonium thiosulfate, as sodium thiosulfate, the treated aqueous streamwill contain ammonium hydrosulfide as sodium hydrosulfide, respectively.In addition, in the case where the cooling step of the present inventionis operated in a manner so that not all of the polysulfide is decomposedtherein, this treated stream may have some minor amounts of residualpolysulfide contained therein. If it is desired to further purify thistreated stream by removing sulfide and polysulfide therefrom, it can beeasily accomplished by any sulfide removal means known to those skilledin the art such as by a suitable stripping operation with an acid gassuch as C0 It is to be noted that in many cases the treated aqueousstream is to be reused in a process where the presence of sulfidecompounds therein is not objectionable; consequently, removal of sulfideis not required in all cases.

EXAMPLE I In order to demonstrate an importance of the presence of H 8during the sulfur separation step of the present invention, twoexperiments were performed in which the effect of H 8 partial pressureon the recovery of elemental sulfur from a freshly decomposedpolysulfide solution was studied.

In the first experiment, a simulated partially decomposed polysulfidesolution was prepared by mixing 80 grams of finely divided elementalsulfur with 260 cc. of an aqueous solution containing 0.016 wt. percentsodium sulfide along with an equivalent amount of sodium bicarbonate.The resulting solution was heated in an appropriate vessel to C. under30 psig. of nitrogen for about one-half hour. A liquid sulfur phase wasthen withdrawn from the vessel and found to contain 43 grams ofelemental sulfur. This result indicated that about 37 grams of theelemental sulfur originally charged to the vessel was hydrolyzed thereinto sodium polysulfide. That is, about 46 percent of the elemental sulfurpresent in the simulated effluent from the polysulfide decompositionstep was lost by back hydrolysis in the sulfur separation step when theelemental sulfur is just simply separated from the treated solution.

The second experiment was conducted according to the method of thepresent invention and essentially involved the addition of 70 psi of H 8to the separation vessel. The experiment repeated above was thenrepeated with this modification and it was found that 72 grams of theelemental sulfur was recovered. Thus the effect of performing thissulfur separation step under an H 8 partial pressure was to increase theyield of elemental sulfur by a factor of about 1.7 or by 67 percentrelative to the control run.

EXAMPLE II heated stream is then raised by a conventional pumping meansto the value of about 790 psig. The resulting heated and high pressureaqueous stream is then admixed with a gas stream containing 91 molepercent H25, 4.8 mole percent CO and 4.2 mole percent H2O. V

The temperature of the gas stream is about 100F and its pressure isabout 780 psig. The i'esulting inixture is then passed into the top of aconventional, vertically positioned reduction zone containing a fixedbed of a reduction catalyst comprising a catalytically effective amountof cobalt sulfide combined with an activated carbon carrier material.More particularly, the catalyst comprised about 2.3 wt. percent cobaltas cobalt sulfide combined with 10 to 30 mesh particles of activatedcarbon. The inlet temperature and pressure of the reduction zone arefixed at about 350 F. and about 780 psig, respectively. In addition, theflow rate of the aqueous solution is adjusted to achieve a liquid hourlyspace velocity of about 1.5 hrs. based on the volume of the catalystbed.

An effluent stream is then withdrawn from the bottom region of thereduction zone and found to be at a temperature of about 392 F. and at apressure of about 770 psig. The positive differential temperature acrossthe reduction zone is of course caused by the exothermic reaction takingplace therein; likewise, the negative differential pressure is caused bythe pressure drop associated with the internals of the reduction zone.The relative rates of circulation of the liquid stream and the gasstream through the reduction zone are adjusted to result in the moleratio of I-I,S to sodium thiosulfate entering the reduction zone perunit time of about 5.6:1. Analysis of the total effluent stream from thebottom of the reduction zone indicates that it contains, 0.6 wt. percentunreacted H2S 0.04 wt. percent C0 1.06 wt. percentsodiurn thiosulfate,1.6 wtj pefcent sodiumbicarbonate and 20.9 wt. percent sodiumpolysulfide.

In accordance with the present invention, the total effluent stream fromthe reduction zone is then quickly passed to a conventional cooling zonewherein its temperature is dropped to about 250 F. while its pressure ismaintained constant at about 7 70 psig. The flow rate of this efi'luentstream and the distance between the re duction zone and the cooling zoneare adjusted so that the cooling of the effluent occurs within 5 minutesafter the effluent leaves the bottom of the reduction zone. This coolingstep corresponds to a reduction in the temperature of the efiluentstream of about 142 F. and is effective to decompose the polysulfidesolution and release substantial quantities of elemental sulfur. It isto be noted that this cooling step occurs under a partial pressure of H8.

The resulting cooled effluent stream, now containing elemental sulfur,is then passed into a conventional sulfur separating zone where a liquidsulfur phase is allowed to separate from the cooled effluent stream. Inaccordance with the present invention the separating zone is maintainedunder a pressure of 770 psig. and a positive partial pressure of I-I,S.

A stream of substantially pure, liquid elemental sulfur is thenwithdrawn from the lower region of this sulfur separating zone. Asupernatant liquid also withdrawn from the upper region of this sulfurseparating zone and passed through a suitable pressure reduction meansto a gas-liquid separating zone maintained at a temperature of about 250F. and a pressure of 50 psig. A gas phase is allowed to separate fromthe liquid phase in this last separating zone and the gas phase iswithdrawn as an overhead stream and the liquid phase comprises thebottoms stream. The overhead gas stream comprises primarily I-I2Sadmixed with some H 0 and CO and it is withdrawn from the separatingzone, admixed with additional makeup H,S and passed back throughsuitable compressing means to the reduction zone in order to provide thegas stream used therein. The liquid phase is likewise withdrawn fromthis last separating zone and it comprises the treated water productstream of the present invention. An analysis of the treated water streamindicates it contains about 1 wt. percent sodium thiosulfate.Calculations based on the disappearance of the thiosulfate indicate thatabout 93 percent of the thiosulfate present in the input water streamcharged to the reduction zone is eliminated by the method of the presentinvention in this particular case.

It is intended to cover by the following claims all changes,modifications and variations of the above disclosure of the presentinvention that would be selfevident to one of ordinary skill in thewater-treating art.

I claim as my invention:

1. A method for treating an aqueous stream containing a water-solublethiosulfate compound to substantially lower the thiosulfate contentthereof andto produce elemental sulfur, said method comprising the stepsof:

a. reacting the aqueous stream with an amount of I-I,S

selected to provide greater than 2 moles of H,S per mole of thiosulfatecontained in the aqueous stream at polysulfide formation conditions,including a temperature above the melting point of sulfur and a pressuresufficient to maintain the aqueous stream in the liquid phase, selectedto form an effluent stream containing the corresponding polysulfidecompound and unreacted H 8;

b. rapidly cooling the total effluent stream from step (a) to atemperature selected to decompose at least a portion of the resultingpolysulfide compound and form elemental sulfur without substantiallychanging the pressure of the effluent stream;

c. subjecting the resulting cooled effluent stream to separationconditions effective to remove elemental sulfur therefrom withoutsubstantially changing the pressure of the effluent stream; and

d. reducing the pressure of the resulting cooled and separated effluentstream and further separating same into an H S-containing gas stream anda treated water stream having a substantially reduced thiosulfatecontent. I

2. A method as defined as in claim 1 wherein the water-solublethiosulfate compound is ammonium thiosulfate. 7

3. A method as defined as in claim 1 wherein the water-solublethiosulfate compound is an alkali metal thiosulfate compound.

4. A method as defined in claim 3 wherein the alkali metal thiosulfatecompound is sodium thiosulfate.

5. A method as defined in claim 3 wherein the alkali thiosulfatecompound is potassium thiosulfate.

6. A method as defined as in claim 1 wherein the water-solublethiosulfate compound is a water-soluble alkaline earth metal thiosulfatecompound.

7. A method as defined in claim 6 wherein the alkaline earth metalthiosulfate compound is calcium thiosulfate.

8. A method as defined in claim 1 wherein the mole ratio of H 8 tothiosulfate compound utilized in step (a) is selected from the rangecorresponding to about 3.5:1 to about 10:1.

9. A method as defined in claim 1 wherein the temperature utilized instep (a) is selected from the range of about 150 to 370 C. and thepressure utilized is selected from the range of about to about 5,000psig.

10. A method as defined as in claim 1 wherein at least a portion of thegas stream formed in step (d) is passed to step (a) in order to furnisha portion of the H 8 reactant utilized therein.

11. A method as defined in claim 1 wherein the temperature utilized instep (b) is about 25 to about C. below the temperature used in step (a).

12. A method as defined as in claim 1 wherein the temperature utilizedin step (b) is just above the melting point of sulfur.

2. A method as defined as in claim 1 wherein the water-solublethiosulfate compound is ammonium thiosulfate.
 3. A method as defined asin claim 1 wherein the water-soluble thiosulfate compound is an alkalimetal thiosulfate compound.
 4. A method as defined in claim 3 whereinthe alkali metal thiosulfate compound is sodium thiosulfate.
 5. A methodas defined in claim 3 wherein the alkali thiosulfate compound ispotassium thiosulfate.
 6. A method as defined as in claim 1 wherein thewater-soluble thiosulfate compound is a water-soluble alkaline earthmetal thiosulfate compound.
 7. A method as defined in claim 6 whereinthe alkaline earth metal thiosulfate compound is calcium thiosulfate. 8.A method as defined in claim 1 wherein the mole ratio of H2S tothiosulfate compound utilized in step (a) is selected from the rangecorresponding to about 3.5:1 to about 10:1.
 9. A method as defined inclaim 1 wherein the temperature utilized in step (a) is selected fromthe range of about 150* to 370* C. and the pressure utilized is selectedfrom the range of about 100 to about 5,000 psig.
 10. A method as definedas in claim 1 wherein at least a portion of the gas stream formed instep (d) is passed to step (a) in order to furnish a portion of the H2Sreactant utilized therein.
 11. A method as defined in claim 1 whereinthe temperature utilized in step (b) is about 25* to about 150* C. belowthe temperature used in step (a).
 12. A method as defined as in claim 1wherein the temperature utilized in step (b) is just above the meltingpoint of sulfur.